Waves and streams in the expanding solar wind

The expanding box model (EBM) allows the simulation of the evolution of compressible MHD turbulence within the expanding solar wind, taking into account the basic properties of expansion. Using the EBM we follow the evolution of waves within a compressive stream shear and magnetic sector structure in the range of 0.1 to 1 AU from the Sun. We analyze the physical processes which lead in these simulations to the modulation and erosion of the wave component, combined with WKB and non-WKB processes due to expansion. A strong erosion by stream shear corresponds indeed to one of the observed regimes in the solar wind; however, we are unable to reproduce the regime which holds during solar minimum, in which the correlation between large-scale stream structure and turbulence remains high independently from distance to the Sun. The main point of disagreement with observations concerns the energy spectrum (it is difficult to generate and sustain small-scale turbulence with an Alfvenic wave band present, and even more so in an expanding medium); the main point of agreement concerns the statistics of density fluctuations, which are independent of distance, and matches the observed amplitudes both within slow and fast wind. At the same time, small scales appear to be dominated in the simulations by compressible effects, which contradicts popular ideas on solar wind turbulence.